Magnetically soft materials, i.e., materials which typically exhibit macroscopic ferromagnetism only in the presence of an applied magnetic field, find application in a great variety of technological fields. Exemplary uses are in heavy-current engineering, transconductor cores, relays, inductance coils, transformers, and variable reluctance devices. Although many alloy materials are known to be magnetically soft, this invention is concerned only with magnetically soft Fe-Cr-Ni alloys, and in particular, Fe-rich alloys, and our discussion will be restricted accordingly.
The Fe-Cr-Ni alloy system comprises a substantial number of technically important compositions. Among them are the structural steels (e.g., 1.5% Cr, 3.5% Ni, and 0.3% C), the stainless steels (e.g., 18% Cr, 8% Ni), the heat-resistant alloys used as furnace elements (e.g., about 15% Cr, 80% Ni), and the Elinvar type alloys having low temperature coefficients of elastic moduli (e.g., 12% Cr, 36% Ni). In addition to these alloys whose magnetic properties are typically of little or no concern, the system also contains Chromium Permalloy (e.g., 3.8% Cr, 78% Ni), used principally in transformers where high initial or reversible permeability is required. Magnetic Fe-Cr-Ni alloys have also been used in applications requiring magnetic properties that change rapidly with change in temperature. A specific alloy, containing 35% Ni, 5% Cr, 0.3% Si, and the remainder iron, has been suggested for use in relays that open or close at required temperatures and in transformers that control the operations of small motors or other equipment. Fe-Cr-Ni alloys are discussed in R. N. Bozorth, Ferromagnetism, Van Nostrand Company, (1951), pp. 146-153, incorporated herein by reference.
As a general rule, a soft magnetic alloy material should in the final product by a single-phase solid solution in its equilibrium state. See, for instance, C. W. Chen, Magnetism and Metallurgy of Soft Magnetic Materials, North-Holland Publishing Company, 1977, page 267, where this is referred to as the "first rule for soft magnetic materials". In agreement with this rule, iron-rich Fe-Cr-Ni alloy compositions have not found use as soft magnetic materials, since, inter alia, in practice they are not easily prepared as single-phase materials. Furthermore, they typically have relatively low mechanical strength in this condition. More generally, Bozorth states that the Fe-Cr-Ni alloys that are useful for magnetic purposes lie in the large region of homogeneous .gamma. solid solution of face-centered cubic structure (austenite). This region, as can be seen from the phase diagram on page 148 of Bozorth (op. cit.), comprises the Ni-rich region of the phase-diagram, with Ni-content of more than about 14%.
Technologically important applications exist for magnetically soft alloys that possess, in addition to the appropriate magnetic properties, relatively high mechanical strength, that are wear resistant and rust resistant. For instance, such an alloy finds use in telephone ring armature applications. See Mott et al, Bell System Technical Journal, Volume 30, January 1951, pp. 110-140. The material currently used in this application is typically 2V-Permendur (49% Fe, 49% Co, 2% V). This alloy is expensive because of its high cobalt content. Furthermore, the alloy typically is difficult to process, requiring a rapid quench to avoid embrittlement due to a order-disorder phase transformation.
Because of cobalt's high cost and uncertain supply status, a Co-free alloy having magnetic properties, mechanical strength, and corrosion resistance similar to, or better than, those of Permendur would be of considerable commercial interest, and could find application in a variety of devices.